Printers such as inkjet printers generally employ printheads which are mounted on a scan axis for printing in a swath across a sheet of a print medium. The print medium, whether or not of paper, may be referred to herein as a “page” for simplicity, although any print-receiving medium is encompassed by this term whether in page format, in the form of an endless web, or in the form of an article such as an envelope which is fed through the printer).
The page is incrementally advanced through the printer in a direction perpendicular to the scan axis (the direction of paper movement is known as the “media axis” or as the printing advance direction or the paper axis directionality or “PAD”, and the terms are used interchangeably herein). Between each incremental advance a swath of ink is deposited on the paper.
When an image is sent to the printer, the printing software generates an image mask in which the image is split into swaths of a height equal to the height of the printhead. FIG. 1 shows a print carriage of the type used in the Hewlett-Packard DesignJet 5000 printer. Six print cartridges 10 are mounted on a carriage 12 which travels along a pair of parallel rails 14 defining a scan axis. The carriage is driven by a belt 16 along the scan axis. The belt is driven by a motor mounted within the printer (not shown) and a set of off-board ink reservoirs feed ink to the individual print cartridges 10 via a set of six flexible tubes (not shown) whereby each printhead can be supplied with a different coloured ink (e.g. dark cyan, light cyan, dark magenta, light magenta, yellow and black).
Each print cartridge 10 supplies ink to a printhead or pen 18 comprising a linear array of nozzles, in this case arranged in two parallel staggered rows 20,22, running in the direction of the media axis.
In use the printer software converts images to be printed into an image mask of pixels of the six different colours. High quality colour hues can be printed by an appropriate mix of coloured dots laid alongside or on top of one another. In a 600 dpi (dot per inch) print mode, therefore, each square inch of the image will be pixelated into a 600×600 grid, and each point of the paper will either be left blank or will receive a droplet of ink from one or more of the pens. The manner in which the droplets are laid down is specified in the print mode.
In a low quality but high speed single pass print mode, the image mask is divided into a series of swaths running parallel to the scan axis. The paper advances in steps in the printing advance direction (PAD). As each successive swath of paper is located under the path of the print carriage, the carriage scans across the scan axis and the individual nozzles within the pens fire in a timed sequence to deposit drops of the relevant coloured inks onto the paper in the positions called for by the image mask.
Thus, whenever a drop of dark cyan ink is specified in this strip, the appropriate nozzle is caused to fire as it passes over that point in the page. The entire swath receives its full image in a single pass and the page is then advanced before the adjoining swath is printed.
Single pass print modes, in particular, suffer from boundary matching—a problem caused by imperfectly fabricated end nozzles on a print head overlapping (or not) from swath to swath. One way to mitigate this problem and also to improve other quality attributes is to use multi-pass print modes explained below.
FIG. 2 illustrates a prior art two-pass print mode and a prior art eight-pass print mode. Dealing first with the two pass print mode, indicated generally at 24, a sheet of paper 26 is fed through a printer (not shown) past print carriage 12, which is shown schematically as an array of six parallel pens 18, in the PAD (indicated by arrow 28). The print carriage 12 is also shown during a second pass 12′ and a third pass 12″. In the first pass shown, the pens 16 print a swath of height with the top of the swath being denoted by dotted line 30 (line 30 is also marked with the number “1” to indicate that this is nominally the first of two passes), and the bottom of the swath being denoted by dotted line 32. The printer software subdivides the pixels forming the swath of image between lines 30 and 32, so that half of the pixels in any given region are printed during one of two passes. Thus, when carrying out this first pass the pens 18 will print half of the image content between the lines 30 and 32.
The paper then advances half a swath length so that the top of the pens (now shown as 18′) in the print cartridge 12′ lie along line 34 (also marked “2” to denote that it is the top boundary of the second of two passes). The print cartridge 12′ traverses the scan axis parallel to line 34 and the pens 18′ print both the second half of the image between the lines 34 and 32 and half of the pixels required for the image in the half swatch length below line 32 (i.e. again the pens 18′ are printing a full swath corresponding to their full height). The paper then advances so that the top of the pens 18″ lie at line 32 and they print a further full-height, half-content swath, completing the image in the half swath length below line 32, together with the first half of the pixels in the half swath length below that. In this way the printhead prints the entire image in swaths, with the lower half of each swath being printed on fresh paper (or whatever other print media is being employed) and the upper half of each swath being printed over part of the image which was printed during the previous pass.
Thus, the lower nozzles of each pen continually print on blank paper in each pass to provide an image with 50% of the required ink droplets, whereas the upper nozzles always print over the 50% completed image printed by the lower nozzles in the previous pass (which, due to the page advance, are located under the upper nozzles having advanced a half swath length).
Although the print carriage is shown at or towards the left of the page 26 for each pass, in practice the carriage will print alternate scans from the left and right to increase throughput by taking advantage of the end point of carriage travel during the previous scan.
FIG. 2 also illustrates a further multi-pass print mode, in this case comprising 8 passes, indicated generally at 36. The carriage has been further simplified for the eight-pass print mode as a block of six pens 18, and is shown in the position relative to the paper for each of eight successive passes (with the position of the top of the pens after each page advance being indicated by the lines 1,2,3,4,5,6,7,8,1).
In a multi-pass print mode, the paper advances between each pass through a fraction of the swath height, in the example ⅛th. In order for the swaths to match up (and since the total number of nozzles may not be equally divided between the number of passes), it is common to cause the paper to advance by a distance corresponding to the length of an integer quotient of the total number of nozzles divided by the number of passes. In this way the remainder number of nozzles are left redundant in this print mode. In each pass the set of six printheads 24 print a full height swath (except the redundant nozzles) as described above but with the important difference that each swath prints only a fraction of the pixels required for the image mask of that swath. Thus each fraction of the swath height, e.g. the distance between any two of the adjacent lines 1,2,3,4,5,6,7,8,1, receives its complete complement of ink droplets over, in this case, eight passes.
The printer control software is therefore adapted, in a known manner, to generate instructions causing the printheads to print a series of swaths each containing a fraction of the image content. This fraction is defined in a print mask setting out the pixels which are to be printed in each pass.
So in a single-pass print mode, all pixels in the mask are employed in each pass. In two-pass print mode, a checkerboard type mask is employed, typically so that diagonally connected pixels are printed in a first pass and their complementary diagonally connected pixels are printed in the second pass. Thus, in each pass a drop is being deposited on a pixel which may have up to four adjacent pixels which have been printed in the previous pass. Four-pass print mode, may either operate by employing a ¼ filled mask and advancing ¼ of the swath length in each pass; or alternatively, by using a two-pass type mask to fire two dots per pixel. The latter option means less ink is deposited in each pass, so drying faster and allowing a saturation amount of ink to be deposited on a given pixel over two passes. It will be seen by extension that in an 8-pass print mode, the mask can be defined so that in any given pass, a drop need not be fired onto a pixel which has been printed in an immediately previous pass, so ensuring that the medium is completely dry when printed on.
In general, a higher number of passes is used to provide a better quality image and, due to the fact that each area of the paper is printed in a number of passes, a higher fidelity image can be obtained. However this normally results in a slower print job, due to the fact that the paper is advancing only a smaller fraction of the active pen height between passes.
Thus, many printer designers will often attempt to implement a high quality print mode by using say a four or six pass print mode rather than 8 pass mode. To attempt to increase throughput in such print modes, it is usual to attempt to employ the fastest possible print head speed over the scan axis, however, as explained below this can result in problems.
In any printer, dot penetration and dot gain affect the perceived color of a printed image. They are highly dependent on the media surface conditions encountered by the ink. Absorption of the ink into the media reduces the light filtering capability of the dot. The more the colorant is confined to the paper's surface, that is, the smaller the dot penetration, the most saturated the color appears due to decreased light scattering and absorption within the media. Conversely, if the colorant is spread through a thick layer—great dot gain—the image loses saturation due to decreased colorant density.
Relaxation time is the time required for ink to dry into a given medium and FIG. 4 illustrates a typical time-dryness function indicated by the numeral 40. In this case, the relaxation time (RT1) is relatively short. The time interval between consecutive drop placements (drop-to-drop interval) is usually determined according to the surface conditions found by a second drop on the same dot or in some cases a drop on adjacent dots.
In multi-pass print modes, ink is usually laid down with a drop-to-drop interval (DT) slow enough to ensure that droplets of any particular colour of ink are directed onto a region of paper which is relatively dry.
Thus, as long as this time exceeds the relaxation time (RT) for the ink-medium, the drops will encounter almost the same conditions along the entire print swath, i.e. a relatively small change in dryness (δd), resulting in an homogeneous color perception. However, especially fast bi-directional print modes involve drop-to-drop intervals less than the typical relaxation times and with a maximum variation in drop-to-drop time (DTδ) for any two drops. When the drying time of the ink is relatively fast, this means that the maximum change in dryness (δ1) any two drops may find on the medium can be quite large. Thus, following drops can find very different initial surface conditions along the swath, and this leads to a variation of dot penetrations and gains along the scan axis, and thus to a changing color saturation. In other words, when the carriage has just finished a swath and turns around to start printing in the opposite direction, it is printing on a relatively wet medium, but as it continues along the same swath, the medium is increasingly relatively dry. The related variation in spot size when printing on wet media vs. dry media constitutes the main cause of degradation in image quality in bi-directional print modes and is referenced herein as differential banding along scan axis (DBASA).
It will he seen that DBASA problems do not arise in single pass print modes as the print head does not deposit ink over an area printed in an immediately previous pass—nonetheless these modes and lower pass print modes suffer from boundary banding and other quality limitations.
For intermediate, multi-pass print modes, the typical solution for DBASA has historically been to increase the number of passes as well as giving the ink the chance to dry before the adjacent dots are laid down onto the page.
Thus, it will also be seen that for very high pass print modes, for example, 8 pass, in any given pass a dot need not be deposited adjacent a dot deposited in an immediately previous pass, thus (at current nozzle densities) DBASA problems do not arise in such print modes.
However, because of the lesser number of passes required, it is always preferable to use fewer passes for the same print quality. Thus, it is preferable to use from 2 to 6 or so pass print modes for high quality printing. If, however, the print head moves quickly along the scan axis, so reducing the drop-to-drop interval, DBASA problems can arise. If the print head is set to move slower, then the throughput advantages of employing fewer passes are lost.
Other alternative solutions include switching to unidirectional print modes with the same number of passes or introducing dry times after each swath to reduce DBASA. Again however, these different approaches are trade-offs rather than a solution, since printing times and throughput are adversely affected.
It is also known to employ N pass print modes with N/2 advances, thus repeating each swath in both print directions to reduce DBASA. Though in principle this solution does not affect the throughput and is extensible to print modes with few passes, it must be balanced against the boundary banding problems prevalent in lower pass print modes.
So eventually, and for a low number of passes, a trade-off has to be made between boundary banding and DBASA, the former being less visible from a certain distance.
Because DBASA becomes evident when drop-to-drop intervals are in the order of magnitude of the paper/ink interaction relaxation times, other attempts to solve this problem have gone into reducing relaxation time. One such alternative comprises forcing ink drying and this has no impact on throughput. Nonetheless, such solutions are difficult to implement mechanically and so are unattractive to produce.
Moreover, as DBASA is primarily an ink/medium defect, it can also be concealed by changing the print medium. Thus, when printing on a medium which has different ink/media characteristics than the most affected coated media, there may be no noticeable DBASA artefact.
So while current solutions have focused on reducing the relaxation times well below the drop-to-drop intervals (dryer, increased number of passes) or delaying the swath in order to encounter homogenous paper conditions, they remain insufficient for increasing throughput conditions with fewer-pass print modes.
European Patent Specification EP 0 979 734 A1 discloses a multi-pass ink jet printer in which a first group of ink injection elements deposit more ink along respective rows of a printing medium during a first swath than that deposited by a second group along the same rows in a subsequent swath. However, the reason for this is to avid color shifts in the case of composite colors, and the specification is completely silent with respect to drop-to-drop intervals and relaxation times as hereinafter discussed.